Turbines generally comprise a turbine wheel mounted in a turbine chamber, an annular inlet passageway arranged around the turbine chamber, an inlet chamber arranged around the inlet passageway and an outlet passageway extending from the turbine chamber. The passageways and chambers communicate such that pressurized gas admitted to the inlet chamber flows through the inlet passageway to the outlet passageway via the turbine chamber, thereby driving the turbine wheel. In a variable geometry turbine, one wall of the inlet passageway is defined by a movable annular wall member the position of which is adjustable, relative to a facing wall of the inlet passageway, to control the width of the inlet passageway.
One known variable geometry turbine arrangement is
described in European Patent Specification EP-A-0080810. In the described arrangement therein, a thin walled annular wall member is supported on a pair of guide pins which extend parallel to and are slidable parallel to the axis of rotation of the turbine wheel. Each pin is acted upon by a respective actuator. Such an arrangement raises various problems in terms of ease of manufacture and reliability. With regard to ease of manufacture, the actuators must be accommodated in the limited space around and close to the axis of the turbine, and this is a real constraint upon the turbine design. With regard to reliability, the structure is exposed to considerable temperature gradients which can lead to jamming of the pins if they are subjected to transverse stress. Doubts as to long term reliability have been a major factor in holding back the introduction of variable geometry turbines.